Various types of wellbore fluids are used in operations related to the development, completion, and production of natural hydrocarbon reservoirs. The operations include fracturing subterranean formations, modifying the permeability of subterranean formations, or sand control. Other applications comprise the placement of a chemical plug to isolate zones or complement an isolating operation. The fluids employed by those operations are known as drilling fluids, completion fluids, work over fluids, packer fluids, fracturing fluids, conformance or permeability control fluids and the like.
Of particular interest with regard to the present inventions are fluids for water control applications: During the life cycle of a hydrocarbon well, e.g., a well for extracting oil or natural gas from the Earth, the producing well commonly also yields water. In these instances, the amount of water produced from the well tends to increase over time with a concomitant reduction of hydrocarbon production. Frequently, the production of water becomes so profuse that remedial measures have to be taken to decrease the water/hydrocarbon production ratio. As a final consequence of the increasing water production, the well has to be abandoned.
In many cases, a principal component of wellbore service fluids are gelling compositions, usually based on polymers or viscoelastic surfactants.
Viscoelastic surfactant solutions are usually formed by the addition of certain reagents to concentrated solutions of surfactants, which most frequently consist of long-chain quaternary ammonium salts such as cetyltrimethylammonium bromide (CTAB). Common reagents which generate viscoelasticity in the surfactant solutions are salts such sodium salicylate and sodium isocyanate and non-ionic organic molecules such as chloroform. The electrolyte content of surfactant solutions is also an important control on their viscoelastic behaviour.
Further references related to the use of viscoelastic surfactants as wellbore service fluids can be found for example in U.S. Pat. Nos.4,695,389, 4,725,372, and 5,551,516.
There has been considerable interest in the viscoelastic gels formed from the solutions of certain surfactants when the concentration significantly exceeds the critical micelle concentration. The surfactant molecules aggregate into worm-like micelles which can become highly entangled at these high concentrations to form a network exhibiting elastic behaviour. These surfactant gels are of considerable commercial interest, including application as oil well fracturing fluids.
The viscoelasticity of the surfactant solutions appears invariably to form rapidly on mixing the various components. The resulting high viscosities of the viscoelastic gels can make handling or placement difficult. For example, placement of a uniform surfactant gel in a porous medium is difficult since injection of the gel in the medium can lead to the separation of the surfactant from the solute by a filtration process. Any application of viscoelastic surfactant solutions which requires their transport or placement after their preparation would benefit from a method of controlling their viscosities and gel times.
The object of this present invention is therefore to provide improved compositions for wellbore service fluids based on viscoelastic surfactants. It is a specific object of the invention to provide means of controlling or delaying the build-up of viscosity in such compositions. It is a further specific object of the invention to provide such compositions for water control operations in hydrocarbon wells.